Hot dipped steel tube and a method for producing the same

ABSTRACT

A hot dip-metal coated steel tube having a high corrosion resistance and an optionally beautiful appearance, and a method and apparatus for manufacturing a continuous length of the said tube from a steel strip automatically in an in-line process is disclosed. A plurality of uniformly distributed dents are formed in the surface of a continuous length of steel strip. The dented strip is formed into tubing and a metal coating is applied to the tubing. The uniform distribution of the small dents on the surface of the tubing permits a heavy metallic coating to be applied to the tubing and improves the bond between the coating and the surface of the tubing, and the appearance of the tubing is improved. The continuous manufacturing system moves the tubing along its length from a welding apparatus through a temperature controlling device, wherein the heat from the hot welded seam is dispersed over the tube circumference to provide an efficient and uniform pickling in a subsequent pickling step without any local over pickling or under pickling on the tube surface, and the oxide film formed at the welded seam of the tube is removed by means of pickling.

United States Patent 1191 Nakamura Dec. 23, 1975 HOT DIPPED STEEL TUBEAND A METHOD FOR PRODUCING THE SAME [75] Inventor:

[73] Assignee: Daiwa Steel Tube Industries Co.,

Ltd., Osaka, Japan 221 Filed: Feb.15, 1974 21 Appl. No.: 442,860

Matsuichi Nakamura, Osaka, Japan Primary Examiner-l-larold D. WhiteheadAssistant Examiner.lames G. Smith Attorney, Agent, or Firm-Jones, Thomas& Askew [57] ABSTRACT A hot dip-metal coated steel tube having a highcorrosion resistance and an optionally beautiful appearance, and amethod and apparatus for manufacturing a continuous length of the saidtube from a steel strip automatically in an in-line process isdisclosed. A plurality of uniformly distributed dents are formed in thesurface of a continuous length of steel strip. The dented strip isformed into tubing and a metal coating is applied to the tubing. Theuniform distribution of the small dents on the surface of the tubingpermits a heavy metallic coating to be applied to the tubing andimproves the bond between the coating and the surface of the tubing, andthe appearance of the tubing is improved.

The continuous manufacturing system moves the tubing along its lengthfrom a welding apparatus through a temperature controlling device,wherein the heat from the hot welded seam is dispersed over the tubecircumference to provide an efficient and uniform pickling in asubsequent pickling step without any local over pickling or underpickling on the tube surface, and the oxide film formed at the weldedseam of the tube is removed by means of pickling.

3 Claims, 7 Drawing Figures U.S. Patent Dec. 23, 1975 Sheet 2 of23,927,816

FIG. 5a

FIG

HOT DIPPED STEEL TUBE AND A METHOD FOR PRODUCING THE SAME BACKGROUND OFTHE INVENTION Hot dip-metal coated steel tubes are normally manufacturedeither by dipping precut tubes in a molten metal or by passing a tube ofa continuous length through a molten metal bath.

In a continuous hot dip-metal coating process of steel sheet or wire,entry into and exit from the molten metal bath usually are made in avertical direction because the sheet or wire can be bent in the bathwith ease. The excess molten metal adhered to the sheet or wire dripsback to the bath as the sheet or wire is drawn in an upward directionfrom the bath and, consequently, does not cause uneven metallic coatingover the surface of the sheet or wire.

In a process in which a steel tube of a continuous length is hotdip-metal coated on its outside surface and then cut to desired length,the steel tube is manufactured from a continuous steel strip and issubsequently hot dip-metal coated in a continuous in-line process. Oncethe tube is formed, it is not bent and is moved horizontally through theseveral stages of the manufacturing process. Dripping of the excessmolten metal from the tube surface results in an uneven metallic coatingover the tube circumference since the tube exits the molten metal bathin a horizontal direction.

In order to prevent uneven coating due to dripping of molten metal, theexcess molten metal should be blown off by an air or gas jet immediatelyafter the tube exits from the molten metal bath. Even with this blowingstep, it has been difficult to form a heavy metallic coating for highercorrosion resistance as the steel tube surface is too smooth to holdenough molten metal without dripping.

SUMMARY OF THE INVENTION This invention relates to a hot dip-metalcoated tube of beautiful appearance and high corrosion resistance, and amethod for manufacturing a continuous length of steel tube from anysteel strip by forming a number of small dents with a depth of severalmicrons to a hundred microns and a diameter of several microns toseveral hundred microns evenly over the surface to receive a heaviermetallic coating as well as improve the bonding strength of the coatinglayer. The resulting product has a uniformly indented surface appearancewith a glossy light silver metallic color.

From the above standpoint, an object of this invention is to produce asteel tube having high corrosion resistance and optically beautifulappearance by forming a number of dents with a depth of several to ahundred microns and a diameter of several microns to several hundredmicrons over the steel tube surface. Uniform distribution of these smalldents enables the tube to hold more molten metal on its surface and toform a thick coating layer with a strong bonding.

Another object of this invention is to provide a method formanufacturing the hot dip-metal coated steel tube.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 to 3 are schematicillustrations of the manufacturing process of the steel tube accordingto the present invention, with FIG. 1 illustrating a complete 2processing line and FIGS. 2 and 3 illustrating variations in the latterportion of the process illustrated in FIG. 1.

FIG. 4 shows dents bored on the steel tube surface.

FIG. 5a is a photograph showing a cross section of the steel tubesurface which has been hot dip galva nized without forming any dents onthe base metal. This figure is representative of the prior art.

FIG. 5b is a photograph showing a cross section of a hot dip galvanizedlayer over an indented steel tube surface in accordance with oneembodiment of this invention.

FIG. 6 is a magnified photograph of a typical galvanized steel tubesurface made by the process of this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS Referring to FIGS. 1 to 3, theapparatus used to perform the process includes an uncoiler 1 for feedinga steel strip 2 from a roll to a shear-end welder 3. A looper system 5is provided for feeding the strip continuously to the system without anyinterruption while the trailing end of the exhausted roll of steel stripand the forward end of a new roll of steel strip are connected togetherby the shear end welder 3. A strip cleaning device 7 receives the stripfrom looper system 5 and removes oil, stain, water, etc. adhered to thesteel strip. The continuous strip 2 is moved along its length from stripcleaning device 7 into shot blast machine 9 which functions to furtherclean the strip and to form a number of dents evenly about the steelstrip surface, said shot blast machine consisting of three compartments.The center compartment 15 is a blast room wherein abrasive blastingwheels 17a, 17b, 17c and 17a, 17'b and 17 'c are respectively arrangedabove and below the path of the steel strip to throw abrasive particlesonto both sides of the strip. The blast wheels 17a, 17c and l7'a, l7'cpositioned near the inlet and outlet of blast room 15 are designed tothrow abrasive particles towards the center of blast room 15 in order toprevent leakage of the abrasive.

The abrasive flow of each blast wheels 17a, 17b, 17c, and l7'a, l7'b,17'c is remote-controlled according to strip feeding speed, stripmaterial, and the surface conditions of the strip. During a transitionperiod of starting or stopping the steel strip, the abrasive flow isregulated by an auto-control system for preventing over blasting andunder blasting.

Front and rear compartments, l3 and 19, respectively, are seal rooms forpreventing abrasive particles from coming out of the shot blast machine9. The rear compartment 19 incorporates a strip cleaner 21 for removalof scale and iron powder on the steel strip as well as for prevention ofthe steel strip carrying out abrasive. The strip guides 11a, 11b, 11cand 11d are positioned inside the shot blast machine 9 and the inlet andoutlet of shot blast machine 9 for guiding the fast moving steel stripthrough the shot blast machine, enabling the strip to have a uniform andefficient shot blasting over its surfaces.

A cold-roll-forming machine 23 receives the strip 2 from shot blastmachine 9 to roll-form the continuous length of steel strip into atubular shape. A seam welder 25 welds along the length of the tubing toclose the tube and form a seam, and temperature controller 27 cools thewelded tube to a proper pickling temperature and distributes the heatfrom the welded seam about the circumference of the tube.

The steel tube is extremely hot at the welded seam after welding and theseam of the tube would carry excessive heat into the subsequent picklingdevice unless it is cooled in the temperature controller 27 prior topickling. The heat distribution over the tube circumference is alsocorrected in temperature controller 27 in order to obtain uniformpickling over the surface. The controller applies water to the tube towash off the iron powder and scale adhered to the tube surface in theshot blasting, roll-forming or welding steps and to prevent pollutionand consumption of acid. At the outlet of temperature controller 27, anair blower 29 is incorporated for removal of water from the steel tubesurface, preventing the pickling solution from becoming diluted withwater.

The continuous tubing is moved along its horizontal path progressivelyto pickling device 31 which removes oxide film from the steel surface,to a water rinse device 33, to a prefluxing device 35 for applying fluxsolution to prevent oxidization of the surface in the subsequent steps,to a drying or preheating device 37, to a hot dip metal coating device39, to a blow-off device 41 which blows air or inactive gas against thetubing to remove excess molten metal adhered to the tube surface and toprevent dripping of the molten metal, to a water quench device 43, to asizing machine 45 for cold-roll-forming the tube to a desired shape orcross sectional dimensions, to a straightener 47 for correcting any bendof the tube, to a surface treatment device 49, and to a tube cut-offmachine 51.

OPERATION The steel strip 2 is fed from uncoiler 1 through looper 5 tostrip cleaner 7 wherein oil, stain and moisture are eliminated. Thestrip which passes through cleaner 7 is fed into shot blast machine 9and subjected to bombardment of abrasive. The particle sizes and theamount of abrasive mixture thrown by blast wheels 17a, 17b, 17c and17'a, 17b, 17'c are determined in accordance -with the surfaceconditions and quality of the steel strip and subsequent coating weightof the metal to be applied to the tubing. The shot blast machine removesthe iron oxide and other contaminants from the continuous steel stripand forms a plurality of dents in the steel strip with a range of depthsfrom a few microns to a hundred microns and a range of diameters fromseveral microns to several hundred microns, and the dents are formeduniformly over the steel strip surface. The abrasive, scale, ironpowder, etc. adhered to the strip surface are removed in seal room 19 bysurface cleaner 21.

The steel strip with a number of dents bored over its surface by shotblast machine 9 is hauled into forming machine 23 and cold-rolled to atubular form, and the seam is welded along the length of the tubing byseam welder 25. Due to weld heat, the seam and the tubing circumferenceare covered with an oxide film with a thickness proportional to thesurface temperature. The thickness of the oxide film is not even overthe circumference of the tubing.

The steel tube is then fed to temperature controller 27 where heatdistribution over the tube circumference is controlled by cooling waterin such a manner that the pickling of the metal will be accomplishedevenly over the surface of the tubing within the same period withoutcausing a local over-pickling or under-pickling. The steel tube with acontrolled surface temperature is fed into pickling device 31 afterremoval of water by air blow-off device 29. As pickling speed isproportional to the surface temperature of the steel tube, the tubeshould be, for an efficient operation, warm enough to shorten thepickling time when it enters pickling device 31. On the other hand, ifthe tube is too warm, it carries excessive heat into the picklingsolution and causes an excessive consumption of acid. The heatdistribution over the tube circumference is also controlled in such amanner that the thicker oxide film portion is warmer than the thinneroxide film portion, enabling the tube to be pickled evenly over thecircumference within a predetermined time. i

The tube is then rinsed by water rinse device 33 and fed to prefluxingdevice 35. Due to the number of dents formed by the shot blast, thesteel tube can hold a heavier flux coating on the surface which willprovide greater resistance to higher temperature conditions for a longertime in the subsequent steps.

The flux-coated tube is then fed to drying and preheating device 37 andsubsequently to the hot dip metal coating device 39, wherein moltenmetal is applied to the surface of the steel tube. The excess moltenmetal on the steel tube is removed by blow-off device 41 to avoiddripping of the molten metal. Since the steel tube has a larger surfacebecause of the number of dents, it can hold more molten metal on thesurface without dripping. Accordingly, a heavier metallic coating isprovided without growth of the alloy layer. The coating layer issuperior not only in corrosion resistance but also in mechanical andphysical properties, since growth of the alloy layer of poor mechanicalproperties is kept small while bonding of the coating layer isstrengthened by the number of dents on the base metal.

In hot dip galvanizing, for example, the base metal, i.e., the surfaceof the tube, has an alloy layer of ironzinc, on which a pure zinc layeris formed. According to this invention, a heavier zinc coating can beaccomplished without growth of iron-zinc alloy since the galvanizedlayer of this invention consists mostly of a pure zinc layer over arelatively thin alloy layer. This will help shorten the dipping time andpermit a higher rate of production by a smaller equipment.

The tube passes through blow-off device 41 where the coating weight iscontrolled and enters water quench device 43. Then it is cold-rolled bysizing machine 45 to a desired shape or cross-sectional dimensions. inthis process, the coating layer which is softer than the base metal issubjected to plastic deformation and bonded more firmly to the basemetal. Thus, the finished coating layer is of a strong and finestructure. Furthermore, the finished surface is a smooth, lustrous, andbeautiful silver-colored aventurine face.

The steel tube is fed to straightener 47 for correction of any bendafter the sizing and fed to cut-off machine 51 and cut to a specifiedlength. I

The method of this invention is summarized as follow-s:

1. The shot blast machine 9 does not incorporate any steel strip driveunit. The steel strip is hauled by forming machine 23 and subjected to aproper and constant tension when it passes through shot blast machine 9,

wherein the abrasive particles are thrown to the strip for forming anumber of dents evenly over the surface. Because of the tension appliedto the strip and the strip guiding system incorporated in the shot blastmachine, vibration and/or twist of the strip is minimized in spite of ahigh speed feeding of the strip. The steel strip processed at the shotblast machine has a uniform dis- 5 tribution of the dents with a depthof several to a hundred microns and a diameter of several to severalhundred microns over its surface.

As illustrated in FIG. 4, the dents formed on the strip differ from eachother from a microscopic viewpoint while the surface roughness of thesteel strip is uniform from a macroscopic viewpoint. In FIG. 4, a and bshow the dents bored by the first particles thrown and shows the dentbored by the second particle thrown against the dent previously bored bythe first particles. The second particle is not likely to deepen thedent but to bore up the dent since the surface is hardened by the firstparticles. Consequently, dents of nearly uniform depth are formed allover the surface as shown in FIG. 6.

2. Mechanical removal by means of a rotary brush or the like has beenone means for removing the oxide film formed at the welded bead and onthe tube circumference. In this existing method, however, a uniform andreliable operation in removal of the oxide film cannot be assuredbecause of wear of the brush or the like. According to the method ofthis invention, the removal of the oxide film is accomplished bypickling while the weld heat remains on the tube.

The temperature of the steel tube surface, which has risen because ofwelding, is controlled by temperature controller 29 to provide the tubewith a proper temperature distribution corresponding to the thickness ofthe oxide film. As pickling speed is proportional to temperature, thetube is uniformly pickled by pickling device 31 in the predeterminedtime without causing local over pickling or under pickling if the tubehas a proper heat distribution.

3. In shot blast machine 9, an abrasive mixture is thrown over thesurface of the steel strip to form a number of dents with a depth ofseveral to a hundred microns and a diameter of several to severalhundred microns evenly on the surface thereof. These small dents play animportant role in the following sequence.

i. In prefluxing device 35, the steel tube can hold a heavier and moreuniform flux coating without dripping on the surface, enabling the tubeto be protected from oxidation for a longer time at higher temperatureconditions in the subsequent steps.

ii. In the hot dip metal coating process, more molten metal is held onthe steel tube surface, enabling the tube to have a heavier metalliccoating without growth of the alloy layer. This coating layer is notonly alloybonded to the steel tube surface but also is bondedmechanically. Further, the coating layer provides an optically beautifulsheen surface of the aforementioned dents formed over the steel tubesurface.

One embodiment of the steel tube produced in the above described processwill be shown hereinbelow.

(Shot blasting conditions) *Quality of steel strip: Hot-rolled steelsheet--- SPl-IT *Surface condition of the steel strip: Strong hot 'rollscale is formed but no rust is seen over the surface.

*Abrasive Particle: Shot S-60 *Abrasive throwing velocity: 88 m/sec*Amount of abrasive thrown: 80 kg/m (Galvanizing condition) *Temperatureof molten zinc bath: 450C 6 According to the above process, dents ofapproximately 20 microns in depth were formed on the steel strip surfaceas shown in the photograph of FIG. 5b and the coating of the steel tubewas 33 microns thick on an average. The comparative photograph of FIG.5a shows the surface of the steel tube manufactured from the cold rolledsteel sheets with no rust on its surface under the same conditions butwithout shot blasting. The comparative steel tube has a coating layer ofapproximately 15 microns in thickness, which is much thinner than thecoating of the steel tube of this invention.

In the production line configuration shown in FIG. 1, if sizing machine45 is moved to the position between water rinse device 33 and prefluxingdevice 35 as shown in FIG. 2, a rough surface finished hot-dipmetal-coated tube suitable for painting can be manufactured.

Another version of production line configuration is the replacement ofsurface treatment device 49 in FIG. 2 with the painting or plasticcoating device 53. This arrangement enables paint or plastic film to beovercoated on the rough surface of the hot dip metal coated steel tubein a continuous in-line process.

I claim: 1. A method of producing a hot galvinized steel tube in acontinuous in-line process comprising:

continuously pulling a length of steel strip from a supply and movingthe strip along its length through a predetermined path and maintainingtension in the strip as it is pulled from the supply;

continuously shot blasting at least one surface of the strip as thestrip is moved along its path at a position along the path where thestrip is in tension;

continuously cold-forming the strip into tubing after the strip has beenshot blasted with the surface of the strip which has been shot blastedforming the outside surface of the tubing;

continuously hot welding the tubing closed along its length as thetubing continues to move along .its length;

continuously cooling the welded tubing and controlling the heatdistribution from the weld seam about the circumference of the tubing tomaintain the welded seam warmer than the remaining portions of thetubing;

continuously acid pickling the exterior surface of the tubing to removethe oxide film from the surface of the tubing;

continuously applying a hot metal coating to the exterior surface of thetubing;

continuously quenching the tubing; and

continually cutting the tubing into lengths of tubing.

2. The method of manufacturing a metal coated steel tube according toclaim 1 wherein the step of shot blasting at least one surface of thestrip comprises forming dents in the strip with a range of depths ofseveral microns to several hundred microns and a range of diameters ofseveral microns to several hundred microns, with the dents beinguniformly dispersed about the surface of the strip.

3. The method of manufacturing a metal coated steel tube according toclaim 1 and further including the step of progressively applying fluxmaterial to the exterior surface of the tubing after the acid picklingstep and prior to the metal coating step.

1. A METHOD OF PRODICING A HOT GALVINIZED STEEL TUBE IN A CONTAINUOUSIN-LINE PROCESS COMPRISING: CONTINUOUSLY PULLING A LENGTH OF STEEL STRIPFROM A SUPPLY AND MOVING THE STRIP ALONG ITS LENGTH THROUGH APREDETERMINED PATH AND MAINTAINING TENSION IN THE STRIP AS IT IS PULLEDFROM THE SUPPLY; CONTINUOUSLY SHOT BLASTING AT LEAST ONE SURFACE OF THESTRIP AS THE STRIP IS MOVED ALONG ITS PATH AT A POSITION ALONG THE PATHWHEREIN THE STRIP IS IN TENSION; CONTINUOUSLY COLD-FORMING THE STRIPINTO TUBING AFTER THE STRIP HAS BEEN SHOT BLASTED WITH THE SURFACE OFTHE STRIP WHICH HAS BEEN SHOT BLASTED FORMING THE OUTSIDE SURFACE OF THETUBING; CONTINUOUSLY HOT WELDING THE TUBING CLOSED ALONG ITS LENGTH ASTHE TUBING CLOSED ALONG ITS LENGTH CONTINUOUSLY COOLING THE WELDEDTUBING AND CONTROLLING THE HEAT DISTRIBUTION FROM THE WELD SEAM ABOUTTHE CIRCUMFERENCE OF THE TUBING TO MAINTAIN THE WELDED SEAM WARMER THANTHE REMAINING PORTIONS OF THE TUBING; CONTINUOUSLY ACID PICKLING THEEXTERIOR SURFACE OF THE TUBING TO REMOVE THE OXIDE FILM FROM THE SURFACEOF THE TUBING, CONTINUOUSLY APPLYING A HOT METAL COATING TO THE EXTERIORSURFACE OF THE TUBING; CONTINUOUSLY QUENCHING THE TUBING; ANDCONTINUALLY CUTTING THE TUBING INTO LENGTHS OF TUBING.
 2. The method ofmanufacturing a metal coated steel tube according to claim 1 wherein thestep of shot blasting at least one surface of the strip comprisesforming dents in the strip with a range of depths of several microns toseveral hundred microns and a range of diameters of several microns toseveral hundred microns, with the dents being uniformly dispersed aboutthe surface of the strip.
 3. The method of manufacturing a metal coatedsteel tube according to claim 1 and further including the step ofprogressively applying flux material to the exterior surface of thetubing after the acid pickling step and prior to the metal coating step.